This invention relates to the testing of optical fibers, and more particularly, to bend testing of such fibers.
Optical fibers are strands of glass fiber processed so that light transmitted therethrough is subject to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass fibers are fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching and other types of damage. As an example of the dimensions, in one configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the fiber plus the polymer buffer is about 250 micrometers (approximately 0.010 inches).
For such very fine fibers, the handling of the optical fiber to avoid damage that might reduce its light transmission properties becomes an important consideration. The fibers may be wound onto a cylindrical or tapered cylindrical bobbin with many turns adjacent to each other in a side by side fashion. After one layer is complete, another layer of fiber is laid on top of the first layer, and so on. The final assembly of the bobbin and the wound layers of fiber is termed a canister, and the mass of wound fiber is termed the fiber pack. When the optical fiber is later to be used, the fiber is paid out from the canister in a direction parallel to the axis of the cylinder.
It has been found by experience that, where the fiber is to be paid out from the canister in a rapid fashion, as for example over a hundred meters per second, the turns of optical fiber must be held in place on the canister with an adhesive. The adhesive holds each turn of fiber in place on the fiber pack as adjacent turns and layers are initially wound onto the canister, and also as adjacent turns and layers are paid out. Without the use of an adhesive, payout of the fibers may not be uniform and regular, leading to snarls or snags of the fibers that damage them or cause them to break as they are paid out.
When the optical fiber is paid out from the canister in a direction parallel to the cylindrical axis of the canister, the optical fiber is bent through an angle, called the peel angle, with a relatively small bend radius as it is pulled away from the fiber pack to which it is adhesively bonded. The peel angle may vary depending upon the peel tension and the geometry of the peeling, but is typically about 30-60 degrees. It is known that bending of the fiber, such as that experienced during payout, reduces the transmission of light through the fiber, and can cause it to fail mechanically.
The processing of optical fibers has progressed to the point that the loss of light energy resulting from a peel bend can be as small as 0.1 decibel (db) or less. It is important to measure such small energy losses in order to fully characterize the fiber, but such measurements can be difficult due to a variety of effects. There is a need for an approach to the testing of optical fibers that permits small bend losses of the optical fibers to be reliably measured. The present invention fulfills this need, and further provides related advantages.